Determining the direction of a sound source with a miniature receiving device is known in the art. Much of this technology is based on the structure of a fly's ear (Ormia ochracea). Through mechanical coupling of the eardrums, the fly has highly directional hearing to within two degrees azimuth. The eardrums are known to be less than about 0.5 mm apart such that localization cues are around 50 nanoseconds (ns). See, Mason, et al., Hyperacute Directional Hearing in a Microscale Auditory System, Nature, Vol 410, Apr. 5, 2001.
A number of miniature sensor designs exist with various methods and materials being used for their fabrication. One such type of sensor is a capacitive microphone. Organic films have often been used for the diaphragm in such microphones. However, the use of such films is less than ideal because temperature and humidity effects on the film result in drift in long-term microphone performance.
This problem has been addressed by making solid state microphones using semiconductor techniques. Initially, bulk silicon micromachining, in which a silicon substrate is patterned by etching to form electromechanical structures, has been applied to manufacture of these devices. Such MEMS microphones have typically been based on the piezoelectric and piezoresistive principles. Many of the recent efforts, however, have focused on fabrication of small, non-directional capacitive microphone diaphragms made using surface micromachining. Such microphones have sometimes been paired together to create a directional microphone system, but have experienced performance problems.
Other attempts at producing miniature directional microphones involve using filters having a slow wave structure with a certain delay time. However, such attempts have been limited to devices that are tuned to a specific frequency or frequency range, i.e., broadband or narrow band. For example, microphones in hearing aids can be tuned to obtain adequate directional detection for human speech, which is typically between a few hundred to a few thousand Hertz (Hz). Other microphones may be tuned to pick up the sound of a whistle at 5000 Hz, for example. The only means of detecting a wide range of frequencies at the same time with such devices would be to couple several microphones together, each tuned to a different frequency. Such an approach is not only costly and impractical, it is likely subject to performance problems as well.
For the reasons stated above, there is a need in the art for a miniature microphone system capable of detecting a sound source location over a wide frequency range.